Summary: For quantitation of PS content in polyvalent conjugate vaccines by nephelometry, the optimum conditions of concentration for PSs and antibodies were determined. Periodate oxidation is an important first step in the conjugation process. Thus, immunochemical characterization of periodate activated PSs can provide an early indication of stability in total conjugation process. For control testing of PS content in polyvalent PS and PS-protein conjugate vaccines, it is important to have (i) reference standard for each type of PS and monovalent conjugate, (ii) reference specific antiserum to each type of PS. High sensitivity of the scattering rate was observed in many types of pneumococcal PSs using antisera, which were prepared in rabbits hyperimmunized with inactivated whole pneumococci. Light scattering rate nephelometry provides an accuarate and reproducible method for quantitative analysis of PS content in monovalent and polyvalent pneumococcal conjugte vaccines. This method can be adapted to measure the free saccharide content in conjugate vaccine after the free saccharide is separated from the PS-protein conjugate by phenyl-sepharose chromatography or another suitable method. In some licensed biologics, containers and diluent for vaccines have been sterilized by gamma irradiation. It has been used for the sterilization and viral inactivation of blood products and elimination of adventitious agents in cell lines. The objective of the present study was to examine the effect of radiation sterilization on the potency of bacterial PS and PS-protein conjugate vaccines as measured by molecular size and mouse immunogenicity. Three different vaccines, Haemophilus b conjugate, pneumococcal type 6B PS, and typhoid Vi PS vaccines were irradiated (25 kGy) using gamma, electron beam and X-ray sources. In general, irradiation increased the value of the Kd, indicating a decrease in the molecular size of the Hib conjugate, pneumococcal type 6B PS and the typhoid Vi PS vaccines. Although the molecular size decreased under gamma irradiation at 24 kGy, the mouse IgG and IgM antibody responses for the Hib conjugate vaccine did not significantly change. The Kd's for the electron beam irradiated samples still met the product specifications for pneumococcal type 6B PS and typhoid Vi PS vaccine.